This invention relates to power plant pollution reduction equipment, and, more particularly, to an apparatus that reacts NOx in a flue gas stream with a nitrogeneous compound such as a source of ammonia in the presence of a catalyst to reduce the NOx level of the flue gas.
In a fossil-fuel power plant, coal, oil, or natural gas is burned in a combustor. The combustion gas, also known as flue gas, heats water to form stream, which turns a turbine-generator to produce electric power. The flue gas stream is passed through an air preheater, such as a rotating wheel heat exchanger, that transfers heat from the flue gas to an incoming air stream that thereafter flows to the combustor.
The flue gas contains solid particulate and pollutant gases such as sulfur oxides, known as SOx, and nitrogen oxides, known as NOx. To remove the NOx, a nitrogenous compound such as ammonia is injected into the flue gas stream. The ammonia reacts with the NOx to form nitrogen and water, reducing the NOx content of the flue gas. The reaction of ammonia and NOx may be performed at high temperature without a catalyst, a process termed "selective noncatalytic reduction" (SNCR), or at lower temperature in the presence of a catalyst, a process termed "selective catalytic reduction" (SCR).
Selective noncatalytic reduction is accomplished by injecting a nitrogeneous compound such as a source of ammonia into the hot flue gas, and permitting the reduction reaction to occur in the flowing gas. Selective catalytic reduction is accomplished by placing catalyst onto surfaces of a stationary selective catalytic reduction assembly in the form of a fixed catalyst bed and/or onto some of the heat exchange elements of the air preheater. Ammonia is injected upstream of the catalytic reduction assembly and the catalyst-coated elements of the air preheater, as required to accomplish the reaction with NOx in the presence of the catalyst.
It is important to accomplish the reaction of the ammonia and NOx in an efficient manner, for maximum possible reaction of both the NOx and of the ammonia. If the reaction is incomplete, either NOx or ammonia (or both) may pass through to the stack gas and be emitted to the atmosphere. Both NOx and ammonia are classified as pollutants, and their emission is to be maintained within legal limits.
It has been observed in some power plants that, no matter how carefully the ammonia addition is controlled, the catalyzed reaction is incomplete and that either excess NOx or excess ammonia, or both, slip through to the stack gas. There is a need for an understanding of, and solution to, this problem. Such a solution would desirably improve the utilization of ammonia so that the catalyzed reaction between the NOx in the flue gas and the ammonia would be more nearly complete. The present invention fulfills this need, and further provides related advantages.